Some spoilers, if tested properly to increase downforce with little to no increase of drag, work well to push down on the back wheels. This downforce on the rear wheels keeps the balance of downforce on the car, so the rear doesnt slip around and end up in front of the... well, front... so basically you cant have the front of your car planted to the ground and have the back flying in the air. Its for stability at high speeds, and I mean 80mph+. Below that it is weight, drag, and sometimes an eyesore
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